SSH suddenly stops working

I tried to ssh to my XO laptop today, but got the following error:

me:~$ssh user@(IP addr of xo_laptop)
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
blah:blah:blah:blah:blah:blah:blah:blah:blah:blah.
Please contact your system administrator.
Add correct host key in /home/USER/.ssh/known_hosts to get rid of this message.
Offending key in /home/USER/.ssh/known_hosts:NUMBER
RSA host key for (IP addr of xo_laptop) has changed and you have requested strict checking.
Host key verification failed.
me:~$

Why? Probably because I upgraded the OLPC laptop.

How to fix it?

1. Use ssh-keygen -R (IP addr) to purge the old key.
2. Connect again like normal ssh user@xo_laptop to install the new (current) key.